LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Ben Gardon <bgardon@google.com>
To: Lai Jiangshan <jiangshanlai+lkml@gmail.com>
Cc: Sean Christopherson <seanjc@google.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm <kvm@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
	Sergey Senozhatsky <senozhatsky@google.com>
Subject: Re: [PATCH] KVM: x86/mmu: Complete prefetch for trailing SPTEs for direct, legacy MMU
Date: Thu, 26 Aug 2021 14:35:26 -0700	[thread overview]
Message-ID: <CANgfPd---jQ-eO3thxu4bvE+1DkuRXyhjfmZ5dLO6OUQdDsOAA@mail.gmail.com> (raw)
In-Reply-To: <CAJhGHyDWsti6JpYmLhoDfxtaxWC3wFeWzM3NWubqmd=_4ENc3Q@mail.gmail.com>

On Wed, Aug 25, 2021 at 3:49 PM Lai Jiangshan
<jiangshanlai+lkml@gmail.com> wrote:
>
> On Thu, Aug 19, 2021 at 7:57 AM Sean Christopherson <seanjc@google.com> wrote:
> >
> > Make a final call to direct_pte_prefetch_many() if there are "trailing"
> > SPTEs to prefetch, i.e. SPTEs for GFNs following the faulting GFN.  The
> > call to direct_pte_prefetch_many() in the loop only handles the case
> > where there are !PRESENT SPTEs preceding a PRESENT SPTE.
> >
> > E.g. if the faulting GFN is a multiple of 8 (the prefetch size) and all
> > SPTEs for the following GFNs are !PRESENT, the loop will terminate with
> > "start = sptep+1" and not prefetch any SPTEs.
> >
> > Prefetching trailing SPTEs as intended can drastically reduce the number
> > of guest page faults, e.g. accessing the first byte of every 4kb page in
> > a 6gb chunk of virtual memory, in a VM with 8gb of preallocated memory,
> > the number of pf_fixed events observed in L0 drops from ~1.75M to <0.27M.
> >
> > Note, this only affects memory that is backed by 4kb pages as KVM doesn't
> > prefetch when installing hugepages.  Shadow paging prefetching is not
> > affected as it does not batch the prefetches due to the need to process
> > the corresponding guest PTE.  The TDP MMU is not affected because it
> > doesn't have prefetching, yet...
> >
> > Fixes: 957ed9effd80 ("KVM: MMU: prefetch ptes when intercepted guest #PF")
> > Cc: Sergey Senozhatsky <senozhatsky@google.com>
> > Cc: Ben Gardon <bgardon@google.com>
> > Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Ben Gardon <bgardon@google.com>

> > ---
> >
> > Cc'd Ben as this highlights a potential gap with the TDP MMU, which lacks
> > prefetching of any sort.  For large VMs, which are likely backed by
> > hugepages anyways, this is a non-issue as the benefits of holding mmu_lock
> > for read likely masks the cost of taking more VM-Exits.  But VMs with a
> > small number of vCPUs won't benefit as much from parallel page faults,
> > e.g. there's no benefit at all if there's a single vCPU.

Yeah, that probably does represent a reduction in performance for very
small VMs. Besides keeping read critical sections small, there's no
reason not to do prefetching with the TDP MMU, it just needs to be
implemented.

> >
> >  arch/x86/kvm/mmu/mmu.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index a272ccbddfa1..daf7df35f788 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -2818,11 +2818,13 @@ static void __direct_pte_prefetch(struct kvm_vcpu *vcpu,
> >                         if (!start)
> >                                 continue;
> >                         if (direct_pte_prefetch_many(vcpu, sp, start, spte) < 0)
> > -                               break;
> > +                               return;
> >                         start = NULL;
> >                 } else if (!start)
> >                         start = spte;
> >         }
> > +       if (start)
> > +               direct_pte_prefetch_many(vcpu, sp, start, spte);

It might be worth explaining some of what you laid out in the commit
description here. This function's implementation is not the easiest to
read.

> >  }
>
>
> Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>
>
> >
> >  static void direct_pte_prefetch(struct kvm_vcpu *vcpu, u64 *sptep)
> > --
> > 2.33.0.rc1.237.g0d66db33f3-goog
> >

  reply	other threads:[~2021-08-26 21:36 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-18 23:56 Sean Christopherson
2021-08-19  4:15 ` Sergey Senozhatsky
2021-08-25 22:49 ` Lai Jiangshan
2021-08-26 21:35   ` Ben Gardon [this message]
2021-09-23 16:27 ` Paolo Bonzini

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=CANgfPd---jQ-eO3thxu4bvE+1DkuRXyhjfmZ5dLO6OUQdDsOAA@mail.gmail.com \
    --to=bgardon@google.com \
    --cc=jiangshanlai+lkml@gmail.com \
    --cc=jmattson@google.com \
    --cc=joro@8bytes.org \
    --cc=kvm@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=pbonzini@redhat.com \
    --cc=seanjc@google.com \
    --cc=senozhatsky@google.com \
    --cc=vkuznets@redhat.com \
    --cc=wanpengli@tencent.com \
    --subject='Re: [PATCH] KVM: x86/mmu: Complete prefetch for trailing SPTEs for direct, legacy MMU' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).